[WIP] Adding Documentation for Enabling External Authentication - Depends on External auth3 pull request.

Adding ManageIQ/Guides for installing and enabling
external authentication via IPA.

Depends on the External auth3 Pull Request https://github.com/ManageIQ/manageiq/pull/32


For an Appliance to leverage an IPA Server on the network,
the Appliance **must** have time synchronization enabled.
This can be done by either configuring NTP inn the Appliance UI,
from Configure-Configuration-Zone-Server-NTP Settings or by using